This invention relates in general to a developing method useful in electrophotography and, more specifically, to a developing method suitable for reproducing reliably continuous tone images in a short developing period.
In electrophotography an electrostatic image is formed on a photoconductive insulating surface overlying an electrically conductive substrate generally by charging the surface and then selectively dissipating the charge by exposure to a pattern of actinic radiation. Whether the latent electrostatic image is formed by this method or another, the resulting charge pattern is conveniently utilized by the deposition of an electroscopic marking material commonly called a "toner" through electrostatic attraction whereby there is formed a visible image corresponding to the electrostatic latent image.
In the developing process toner is electrostatically attracted and deposited on the latent electrostatic image if its charge polarity is opposite to the polarity of the image, or it is repelled if the polarity of the toner is the same as that of the latent electrostatic image, and deposits on the non-charge areas. Where it is desired to reproduce an image having a high contrast or a line image, the cascade development method is usually employed. If it is desired to reproduce a continuous tone image, electrode development is usually employed in which the conductive electrode is positioned closely against the surface bearing the latent image. The development electrode serves to attenuate the electric field strength above the surface bearing the latent image and by causing the electrical lines of force to straighten above the electrostatic latent image, a uniform field results which allows for higher efficiency and development and good image reproduction without edge effects. In the development method using Coulomb's force of attraction, commonly referred to as attraction development, the potential of the development electrode is preferably kept at the same potential of the conductive substrate of the photoconductive insulating layer for as long a period as possible during the development process. This requirement is very important in order to obtain high quality reproduction. It is found that when the potential difference between the development electrode and the conductive substrate is noticeable, the reproduction of an original image having areas of low image density is poorly reproduced and smoothness of tone suffers such that tone as is obtainable in the silver halide photography process is not approached. One explanation which may be offered in understanding the above phenomenon is that during the development process when toner particles, for example, having positive charges move towards a latent image having negative charges and deposit thereon, the positive charges located at the interface between the photoconductive insulating layer and the substrate migrate to the ground in order to neutralize completely the surface charges of the latent image on the insulating layer. When the electric resistivity between the substrate and the backing electrode or the ground is negligibly small, the charges at the interface discharge instantaneously and the voltage which occurs across the resistive member is also negligibly small, consequently, the development process may be performed without any problem. However, if the resistivity of the substrate is higher than that of the electrode, for example, than that of a metal where, for example, the substrate is a sheet of conductively treated paper, there exists a considerably high floating or stray resistance between the interface and the ground so that an unfavorable voltage is present. In the case of a paper substrate comprising fibers, the voltage is developed because of point to point contact between the substrate and the backing electrode. This voltage is seen to become higher when liquid development is employed in order to accomplish development in a shorter period of time. This bias voltage that develops on the substrate interferes with the deposition of toner particles on the surface bearing the latent electrostatic image. If a bias voltage is developed at about 5 volts, the image areas having a charge less than 5 volts may not be developed initially. Continuing the development process for long periods of time allows deposition of toner particles onto the image areas having low voltage if the life of the latent electrostatic image is long enough to allow development. In actual practice, however, it is found that the latent electrostatic image decays rapidly during the development process so that low surface voltage areas remain undeveloped resulting in poor reproduction of continuous tone images. It, therefore, becomes necessary to maintain the floating resistance between the substrate and the backing electrode as low as possible. Decreasing this floating resistance may be achieved by, for example, by decreasing the resistivity of the substrate itself and/or decreasing the contact resistivity between the substrate and the backing electrode. Therefore, when it is desired to obtain high quality reproduction of continuous tone images in high speed electrophotography, especially when employing liquid development, it is exttremely important to keep good electrical contact between the interface of the photoconductive insulating layer and the substrate and the development electrode. One method to improve such electrical contact is to contact one part of the substrate with the ground conductor when the substrate is a highly conductive member such as a metal. In the situation where the substrate is a flexible one such as a sheet of paper, a special structure is needed to provide good electrical contact. A sheet of paper with a metal foil backing is ideal from the electrical point of view, but its mechanical properties are so different from those of the paper that it has many undesirable points. Carbon impregnated sheets possess low electrical resistivity, however, they are black in appearance so that it is necessary to employ a thicker photoconductive insulating layer or a white undercoating between the photoconductive layer and the substrate. In such a structure it is found that the resistivity of the carbon impregnated paper is higher in several orders of magnitudes than that of the metal sheet so that it is preferable to contact all or most of the back area of the substrate with the backing electrode in order to ground the photosensitive paper sheet. Insufficient contact between the backing electrode and the substrate increases the electric current density resulting in an increased bias voltage which undesirably effects the development process. Maintaining proper electrical contact in order to avoid development of undesirable bias is found to be unexpectedly difficult in such members. It is found in addition where light scattering layers are employed the resistivity of these layers are usually in the order of 100 to 1000 times higher than the carbon impregnated layers so that even if the light scattering layer which is employed is as thin as 1/100 of the substrate thickness impracticable resistivities are still realized.